It is well known in the art that applying a controlled DC current to a lead acid, or similar chemical base, storage battery is a proven, efficient way of charging. That is, the application of charging current effects chemical changes in the battery so that chemical energy is stored. This energy is ultimately converted back to electrical energy when the battery is connected to supply electrical power to a machine or other device.
One detrimental effect of applying charging current in this type of battery is the formation of hydrogen bubbles in the battery electrolyte. This occurs when the application of charging current causes the battery voltage to increase to the gassing voltage, i.e. the voltage at which "gassing", or hydrogen bubble formation, begins in the battery electrolyte. As is known in the art, this condition, which results from a chemical reaction in the electrolyte, is usually indicative of an overcharging rate.
The bubbles tend to accumulate and adhere to the battery plates. The bubbles form a resistive layer on the plates, thereby causing an increase in the internal resistance of the battery and a proportional reduction in charging efficiency. More specifically, the increased resistance absorbs a portion of the charging current. As a result, the battery is heated internally by self-discharge, the current dropping precipitously across this artificial internal resistance. If excessive gassing continues, such that a heavy layer of hydrogen bubbles covers the plates, further application of charging current accelerates internal heating of the battery and the plates. Eventually, the plates overheat sufficiently to warp and short-out the battery.
Attempts have been made in the prior art to avoid the gassing problem. One of the more successful charging systems is disclosed in U.S. Pat. No. 4,740,739 to Quammen et. al, assigned to the present assignee. In this system, discharge pulses are intermittently applied with the charging current. More specifically, the negative current pulses are superimposed over the charging current to produce turbulence in the battery electrolyte. This turbulence serves to actively stir the electrolyte causing advantageous circulation of the electrolyte and scrubbing of the hydrogen bubbles from the battery plates, thereby avoiding the problem outlined above.
It should be appreciated that while this approach successfully prevents bubble accumulation on the battery plates, it does not prevent the formation of hydrogen bubbles. Specifically, even after the battery voltage reaches the gassing voltage, the level of charging current is not varied in this prior arrangement. This results in significant gassing, i.e. hydrogen bubble formation. Accordingly, it should be recognized that the approach disclosed in the Quammen et al patent effectively relieves the problem caused by gassing, but does not substantially limit gassing itself.
Past attempts to eliminate or greatly reduce gassing while still providing an effective charge in a reasonable period of time have not proven particularly successful.
U.S. Pat. No. 4,146,830 to Foster discloses an apparatus that repeatedly incrementally steps down charging current after the battery voltage reaches a target voltage. The target voltage is initially defined by the gassing voltage. However, each time the charging current is incrementally stepped down, the target voltage is also incrementally stepped up. As a result, after the initial charging increment, the target voltage level continues to increase and remains greater than the gassing voltage throughout a large portion of the charging cycle. In effect, the charging current continues to be incrementally reduced from the initial level, but the battery voltage is allowed to reach and surpass the gassing voltage to ever increasing levels. As a consequence, significant detrimental gassing still occurs, and this approach fails to successfully solve the problem.
Similarly, U.S. Pat. No. 4,052,656 to Lavell et al discloses an approach wherein charging current is gradually decreased as charging progresses. However, charging current is not significantly reduced after the gassing voltage is reached and, accordingly, charging current of relatively high level is still applied after the battery voltage reaches and exceeds the gassing voltage. Thus, use of this approach allows an even higher degree of detrimental gassing to occur, i.e. hydrogen gas bubbles are caused to form at an increasing rate, even after the current reduction in the earlier part of the cycle.
Finally, the IUI concept, described in U.S. Pat. No. 4,146,830 at column 1, lines 29-46, is a charging procedure that maintains battery voltage constant at its gassing voltage during charging current reduction. The control for such a system is very complex requiring very finely tuned regulation of the charging current to maintain the battery voltage at a constant voltage. Furthermore, an important IUI characteristic feature as described terminates charging of the battery by simply using a timer. It does not use a feedback device, such as a battery voltage detector, to sense when the battery is fully charged. Accordingly, in many instances overcharging occurs resulting in an inefficient waste of energy and possible damage to the battery, while in other situations the battery may not be completely charged resulting in poor battery performance.